A portion of the disclosure of this patent document contains material which is subject of copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. Copyright 1993 U.S. Robotics, Inc.
1. Field of the Invention
This invention relates to electronic communications systems and more particularly to a system for enabling a computer to transmit and receive information over an analog communications link.
2. Background of the Invention
Computers typically use modems to communicate digital information over voice-grade telephone lines. Such modems translate digitally expressed information from the computer into analog tone signals suitable for transmission over the voice-grade telephone facility, and convert such tones back into digital form when received from the telephone line.
High speed modems may advantageously employ digital signal processing techniques for translating outgoing digital data into a sequence of digital values each representing a desired analog output signal sample amplitude. These digital sample values may then be converted into analog form by a digital-to-analog converter for transmission over the telephone facility. Correspondingly, at the receiving station, the incoming analog signal may be converted into a train of digital sample amplitude values which are then processed to reconstruct the original digital data.
The processing of the digital sample values is complex and has heretofor been accomplished by one or more dedicated microprocessors which form the heart of the digital modem. For example, the 9600 baud HST modem manufactured by U.S. Robotics Corporation and described in U.S. Pat. No. 5,008,091 issued on Apr. 16, 1991 employs three microprocessors: (1) a transmitting microprocessor dedicated primarily to the translation of digital data into digital sample values; (2) a receiving microprocessor devoted primarily to the translation of sample amplitude values back into digital data; and (3) a supervisory microprocessor which serves as the interface to the computer to which the modem is connected.
The present invention, like the digital modems described above, employs analog/digital conversion methods to convert received analog signals into digitally expressed analog sample values and, during transmission, to convert digitally expressed sample values into analog form. Unlike prior digital modems, however, in the present invention the digital sample value signals are not processed by a separate processor or processors within the modem unit, but are rather processed by the microprocessor already present in the connected computer. As a consequence, the cost of the modem is substantially reduced because the need for separate processors is eliminated.
The arrangement contemplated by the invention is implemented by the combination of a conventional host computer employing a microprocessor and a low-cost interface unit consisting of telephone line adaptor circuitry, an analog/digital converter, and a direct digital interface to the host computer""s system I/O bus. The telephone interface unit exchanges digitally-expressed analog sample amplitude values directly with the connected host computer, and the microprocessor within the host computer handles the remainder of the digital processing.
When operating as a modem, digital data can be processed into a sequence of digitally expressed sample values in accordance with a selected one of several accepted modem formats and transmission speeds. Given processors of speeds typically available, such modem processing can take place in real time to eliminate the need for storing the digital sample data in memory; for example, using the preferred embodiment of the invention to be described, a relatively slow Intel 80286 (IBM-AT class) processor operating at 12 mhertz has been shown to have adequate computational capability to perform the modulation and demodulation processing needed for 2400 baud full-duplex modem operation.
Without requiring any additional hardware, the system may be programmed to send and receive graphical data as standard V.29 format facsimile data, or to send and receive data synchronously operating, for example, as an IBM 3270 compatible terminal device. Moreover, when the digital sample values represent conventional voice transmissions rather than data, the system may be used to provide automated voice recording and playback functions to implement a telephone answering, voice message handling, and telemarketing systems.
These and other functions may be added or revised without requiring any hardware modifications by simply changing standard programs which execute on the host processor in standard ways.
These and other features and advantages of the present invention will become more apparent by considering the following detailed description of a preferred embodiment of the invention. In the course of this description, reference will frequently be made to the attached drawings.